The present invention relates to an apparatus and method for measuring the variability of cardiovascular parameters.
The heart rate fluctuates spontaneously about its average value in several frequencies. These fluctuations are attributed to the activity of the two branches of the Autonomic Nervous System (ANS): the sympathetic and the parasympathetic nervous systems. Power spectrum analysis revealed fluctuations in three main frequency ranges: the high frequency fluctuations in the respiration rate, which are related to the parasympathetic nervous system; the mid-frequency fluctuations, which are usually related to both branches of the ANS and the low frequency fluctuations, at the rate of 0.03-0.08 Hz, which are attributed to the activity of the sympathetic nervous system.
The Heart Rate Variability (HRV) measurement provides information on the ANS function: in several pathologies the HRV pattern is abnormal. The measurement itself is simple and noninvasive and has potential to be used as a clinical tool for the assessment of pathological ANS. At present, such measurements are not used routinely because the difference between pathological and normal HRV is not well defined. The pattern of the HRV differs greatly even between normal subjects, and masks the different pattern of the pathological HRV.
During systole (heart contraction), blood is ejected from the left ventricle into the peripheral organs, thereby increasing their blood volume. The measurement of this Systolic Blood Volume Increase (SBVI) is called plethysmography. The simplest plethysmographic method is Photoplethysmography (PPG), in which light is incident on some site of the skin, so that part of it enters the tissue. That light is partly scattered and partly absorbed by the red blood cells. The light which emerges out of the skin is measured by a photodetector. The output signal shows pulsations in the heart rate, due to the variations in tissue blood volume, which occur by the heart beats.
The PPG method is not suitable for absolute evaluation of the SBVI, because the absolute value of the signal depends on the skin color, on the pressure of the probe on the skin and because the signal varies spontaneously as a function of time even during the same examination. At present, the method is used for the measurement of the heart rate (where the absolute PPG is not important) and for pulse oximetry--measurement of oxygen saturation in the arterial blood, by measuring the ratio of the PPG signal for two or three different wavelenths (where only the ratio between two wavelengths is required).
U.S. Pat. No. 4,834,107 discloses a system which, in some respects, is similar to the present invention. In that patent, the PPG signal of one, single pulse, is digitally analyzed, in order to determine the systolic, diastolic and mean blood pressure and the pulse pressure.